Internet Control Message Protocol (ICMP)

1
Internet Control Message Protocol (ICMP)
Chapter 23
2
ICMP

• Usually sent in RESPONSE to an IP Datagram
• Some ICMP's messages are not Responses but are
  Originated as ICMP REQUESTS or ADVERTISEMENTS

3
ICMP Header
4
Echo Request/Reply(Ping)
ICMP Type Code
Description 0 0
Echo reply (ping) 8
0
Echo request (ping)
5
Traceroute

• Sends a series of ordinary datagrams to the
  destination
• Sets the TTL to 1, 2, 3....
• Nth router discards the datagram and returns an
  ICMP message Type 11, code 0, to the source.
• ICMP message includes the IP address of the
  router sending the message

6
ICMP Messages
ICMP Type Code Name 0
0 Echo Request 3
0 Destination network
unreachable 3 1
Destination host unreachable 3
2 Destination protocol
unreachable 3 3
Destination port unreachable 3
6 Destination network unreachable
3 7 Destination
host unreachable 8 0
Echo Reply 11 0
TTL expired 12 0
IP Header bad
7
ICMP Ecapsulation
Note An ICMP message is never generated for an
error caused by an ICMP Message
8
Path MTU Discovery

• Mark the FLAGS field in the IP header with "Do
  Not Fragment" bit set
• If the datagram is too large for the route the
  router will discard the datagram and return an
  ICMP message, "fragmentation was required but not
  permitted".
• Send successive datagrams with smaller and
  smaller sizes until datagram succeeds.
• Reduces CPU requirements on routers